1. Field of the Invention
The present invention relates to hydroxypropylmethylcellulose having a high thermal gel strength, a method for producing the same and a food comprising the hydroxypropylmethylcellulose.
2. Description of the Related Art
Cellulose ether capable of thermoreversibly gelating, such as methylcellulose and hydroxypropylmethylcellulose, is used in processed foods. For example, such cellulose ether is known to improve shape retention during heating when added to dry cheese (JP 01-080252A). The cellulose ether, when added with agar to freeze-dried foods such as freeze-dried tofu (bean curd), can produce a food which maintains its shape, even if large, during rehydration and during cooling after rehydration and has an excellent texture (JP 2005-348721A). The cellulose ether, when added to frozen foods, can thermally gelate during food processing or during heat-thawing to retain water, thereby keeping flavor (JP 2002-51758A).
In general, an aqueous methylcellulose solution forms a harder gel by heating than the gel an aqueous hydroxypropylmethylcellulose solution forms. Thus, methylcellulose is more preferable than hydroxypropylmethylcellulose as an additive for improving shape retention by thermal gelation. However, such methylcellulose has the problem that in the preparation of its aqueous solution, the methylcellulose cannot be dissolved completely unless it is temporarily cooled to 15° C. or lower. By contrast, hydroxypropylmethylcellulose can be dissolved in water at room temperature of 25° C. or higher. Thus, development of hydroxypropylmethylcellulose having excellent solubility and thermal gel strength has been demanded because cooling procedures at 15° C. or lower are often difficult to be performed in food preparation.
In general, the thermal gelation temperature of hydroxypropylmethylcellulose is higher than that of methylcellulose. A low thermal gelation temperature can offer rapid gelation during heating, while an excessively low thermal gelation temperature requires much time for returning to a solution state by natural cooling after temporal thermal gelation. For example, a compound having an excessively low thermal gelation temperature is added to heat-processed foods such as croquette, the food textures may deteriorate because the foods remain gelled during eating and are therefore too hard. Thus, a thermal gelation temperature, in addition to thermal gel strength, must also be taken into consideration in adding hydroxypropylmethylcellulose to processed foods for the purpose of exerting thermoreversible gelation effect.
Hydroxyalkylmethylcellulose having a high thermal gel strength is described in JP 2008-285673A. The hydroxyalkylmethylcellulose is produced through the steps of: adding a hydroxyalkyl etherification agent, and reacting the hydroxyalkyl etherification agent with alkali cellulose and then adding a methyl etherification agent to the reaction product; or the step of adding a hydroxyalkyl etherification agent and a methyl etherification agent such that 40% by weight or more of the stoichiometric amount of the methyl etherification agent is unreacted at the point in time when 60% by weight or more of the stoichiometric amount of the hydroxyalkyl etherification agent is reacted. However, this hydroxyalkylmethylcellulose, when used in food application, has been requested to be further improved in terms of thermal gel strength and a thermal gelation temperature.